This invention relates to seals for use with compressible fluids such as gases. There are many applications wherein housings are provided with interior sections having rotating parts therein, wherein one of the interior housing sections must be isolated from another by means of a seal system cooperating with the rotating part. Such rotating part may be a shaft or a component mounted on a rotating shaft for applications such as gas turbines, fluid pumps, or compressors. Seals for fluids in such applications may comprise circumferential seals or face-type seals.
"Circumferential Seal" is a name describing a generic type of sealing device used widely, inter alia, on aircraft engine applications. These seals consist primarily of several arcuate carbon and/or graphite segments arranged circumferentially to form a continuous relatively stationary sealing ring around the periphery of a rotating shaft. The segment ends contain overlapping tongue and socket joints to restrict leakage at the end gaps. For the aircraft engine sump seal application, these seals are used to separate ambient areas of high pressure air from an oil wetted area at lower ambient pressures and serve two major functions: (1) prevention of oil leakage from the lower pressure compartment, and (2) minimization of the flow rate of hot air from the high pressure area to the oil wetted compartment.
In order to achieve the long wear life demanded in modern aircraft engine or steam turbine applications, it is necessary to reduce the contact forces (unit loads) between the stationary seal carbon segments and the rotating shaft race. At present, state-of-the-art circumferential seals for compressible fluids are limited to pressure drops across the sealing face on the order of 20 to 40 psi in order to meet wear life requirements. In efforts to increase the pressure range of these seals, much work has been expended in the application of hydrodynamic gas bearing technology (normally called "Rayleigh pockets bearing") to the rubbing face of the carbon seal segments. In such devices, a pressure rise is generated by the velocity shearing gradient between the rotating shaft and stationary carbon elements. This pressure rise, acting against the bearing area on the surface of the seal segment, generates a force which is opposite in direction to the rubbing force generated by the ambient pressure drop. This effectively reduces the rubbing loads and increases the pressure range capability of the seal. The major problem with Rayleigh pockets is their very shallow depth (normally in the range less than 0.001 inches). This inherent shallowness does not allow sufficient latitude to prevent wearing away the gas bearing during periods when surface speed is too low to generate sufficient hydrodynamic forces, or when centrifugal inertia, pressure, and thermal gradients distort the rubbing interfaces and result in loss of gas sealing capacity. In other words, the Rayleigh pocket gas bearings are destroyed when 0.001 inches wear occurs. Therefore, the shallowness of these bearings pockets, combined with the loss of wearing surface area caused by introduction of the pockets, has limited the acceptance of this configuration as a viable candidate for long-life high pressure seal applications.
An attempt has been made to increase the depths of the pockets in a Rayleigh-type system by increasing both the depth and length of the pocket and providing a connecting exhaust pocket for the fluid in the Rayleigh groove of even deeper dimension. This can be seen in U.S. Pat. No. 5,145,189, issued to Adam N. Pope on Sep. 8, 1992. While the Pope solution to the elimination of the shallow Rayleigh pockets may be successful in the sense that the longer pockets of Pope are deeper than the normal Rayleigh pockets, it will be noted that a significant amount of excess precise machining must be performed to create the seal components. Note that the Pope design must provide not only a longer Rayleigh-type pocket, but also an exhaust pocket of an even greater depth, and a transverse communication slot between the Rayleigh pocket and the exhaust pocket, which must also be machined.
It is the specific intention of this invention to eliminate such complex and expensive machining while achieving a long-lived seal for fluids having substantially deeper lift pockets than those of the normal Rayleigh design. At the same time, this invention provides a seal with very high pressure differential levels, long life, lower specific rubbing loads for equal radial forces, and therefore lower heat generation rates than those of both Pope and the prior Rayleigh design. In addition, bore sealing dam widths can be increased to offer a more rugged structure to prevent handling and assembly damage when compared with the otherwise more fragile structures of the prior art. Also, the force-generating pockets of this invention are more uniformly distributed across the bore of the seal rings, thereby accommodating any conicity of a shaft race.
For purposes of this description, a primary sealing surface is provided on both a circumferential seal embodiment and a face seal embodiment. Where the primary sealing surface is of constant or single diameter, it is referred to herein as a bore surface or bore region, particularly in connection with circumferential seals. For face-type seals and generic usage, the term sealing surface or sealingface will be utilized.